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C# Random class provides functionality to generate random numbers in C#. The Random class can also generate other data types including strings. In this code example, learn how to create a random number in C#.

By default, the .NET framework generates random numbers based on the current system time. Numbers generated using the Random class are not considered reliable for high-stake scenarios such as cryptography because the system clock has limited granularity. For example, two Random instances created withing couple of milliseconds yield the same sequence of values.

The Random class has a constructor that accepts the seed value. Seed is a a starting value for the pseudo-random number generation algorithm. If we use the same seed for different Random objects, they will generate the same series of random numbers. This functionality is useful when we want reproducibility.

Random permutations can be generated using Fisher-Yates shuffle algorithm. Time is taken proportionally to the toal number of items being shuffled and shuffles them in place. The algorithm swaps the element at each iteration at random among all remaining unvisted indices, including the element itself.

private static Random rng = new Random();

public static void Shuffle<T>(this IList<T> list)
{
    int n = list.Count;
    while (n > 1)
    {
        n--;
        int k = rng.Next(n + 1);
        T value = list[k];
        list[k] = list[n];
        list[n] = value;
    }
}

A very similar algorithm to Fisher-Yates which produces a shuffled version of input is Sattolo's algorithm. Instead of picking an element at random to swap with, like in Fisher-Yates, it picks an element at random that is not the element being placed, i.e it does not allow an element to be swapped with itself. One side effect of this is that no element ends up where it originally started.

private static readonly Random Rand = new Random();

void sattoloCycle<T>(IList<T> items) 
{
    for (var i = items.Count; i-- > 1;) 
    {
        int j = Rand.Next(i);
        var tmp = items[i];
        items[i] = items[j];
        items[j] = tmp;
    }
}